Calcium and bone health
Almost every country mentions a daily requirement for sufficient daily calcium intake for the maintenance of bone health. When consuming soya products as an alternative to cow’s milk products, it is important to choose the calcium-enriched ones.

Nevertheless, it has been established that the countries in which the highest calcium intakes are measured, are also the countries where the number of hip fractures, mostly occurs. It is therefore clear that a number of other factors have an effect on bone health (1-3).

As is the case with cow’s milk, calcium-fortified soya products contain approx. 120 mg calcium for every 100 ml of product. A recent study demonstrated that a daily consumption of three portions (The American Dietary Guidelines 2005 recommend a daily consumption of approx. 710 ml of milk products.) of milk products (approx. 710 ml) provides 855 mg of calcium and 186mg absorbable calcium. If one consumes the same amount of calcium-fortified soya milk, this results in a total of 1,104 mg of calcium, of which 200 to 233 mg are absorbed by the body (4). This study clearly showed that cow’s milk and calcium-fortified soya milk are equivalent sources of calcium.

Isoflavones and bone health
Certain components (isoflavones), which occur naturally in soya, may be beneficial in the case of bone health. Isoflavones have a chemical structure similar to that of oestradiol (endogenous estrogen), are thought to be of benefit for bone health. Based on evidence from 17 in vitro studies of cultured bone cells, 24 in vivo studies of animal models for postmenopausal osteoporosis, 15 human observational/epidemiologic studies, and 17 dietary intervention studies, Setchell and Lydeking-Olsen concluded in 2003 that diets containing phyto-estrogens have bone-sparing effects in the long term (1). An intervention study in 66 postmenopausal women consuming 40 g/d of soy protein that provided 90 mg/d of aglycone isoflavones for 6 months, reported significant net savings in lumbar spine bone mineral content and bone mineral density of 3.5% and 2.8%, respectively (2). In another study, similar anti-osteoporotic effect of soy isoflavones was reported in 69 perimenopausal women receiving 80 mg/d of aglycone soy isoflavones for 6 months (3). In a study reported by Chen et al in 2004, 203 women were randomly assigned to three treatment groups (placebo, N=67; 40 mg aglycone isoflavones, N=68; and 80 mg aglycone isoflavones, N=68) (4). Favorable effect of isoflavone supplementation on rates of change in bone mineral content at the total hip and trochanter was reported among later postmenopausal women (> 4 y), in women with lower body weight (< 55.5 kg), or among women with lower level of calcium intake (< 1,095 mg/d) after one year of treatment. However, no beneficial effect was reported for bone mineral density.

Lydeking-Olsen et al also carried out a 2-y randomized, placebo-controlled trial in 2004 to determine the potential benefit of soymilk to prevent bone loss in postmenopausal Caucasian women with established osteoporosis or at least three risk factors for osteoporosis (5). In the study, 23 women received soymilk that delivered 76 mg/d of aglycone isoflavones, 22 women received transdermal progesterone, 22 women received the soymilk with isoflavones together with the transdermal progesterone, and 22 women received soymilk with low isoflavone content (1 mg/d) or progesterone-free-cream. At the end of two years, bone mineral content and bone mineral density of the lumbar spine were protected among the women receiving the soymilk with isoflavones or the transdermal progesterone. Significantly losses in bone mineral content and bone mineral density of the lumbar spine were observed among the placebo group and among the women receiving the combination treatment. In a large prospective cohort study of soyfood consumption and risk of bone fracture among 24,403 postmenopausal women in the Shanghai Women’s Health Study, Zhang et al reported the incidence of bone fracture over 4.5 year was found to be inversely related to quintiles of soy protein intake (p < 0.01 for trend) suggesting the potential long-term benefit of soy intake against osteoporosis (6).

Conclusion
In 2008, two meta-analysis reviewing the effect of soy isoflavones on bone health were published (7,8). The first meta-analysis combining the results of nine studies with a total of 432 subjects found that isoflavone intervention significantly inhibits bone resorption and stimulates bone formation (7). These favorable effects occur even if <90 mg/day of isoflavones are consumed or the intervention lasts less than 12 weeks. The second meta-analysis of 10 studies with a total of 608 subjects found that the spine bone mineral density in subjects who consumed isoflavones increased significantly by 20.6mg/cm2 in comparison to that in subjects who did not consume isoflavones. Isoflavone intake versus placebo intake increased spine bone mineral content by 0.93g with borderline significance. Increases in the spine bone mineral density with isoflavone intake of more than 90mg/day and with treatment lasting 6 months were 28.5mg/cm2. Thus isoflavone intervention significantly attenuates bone loss of the spine in menopausal women. These favorable effects become more significant when more than 90mg/day of isoflavones are consumed. And soy isoflavone consumption for 6 months can be enough to exert beneficial effects on bone in menopausal women.

1. Dempster DW et al. Pathogenesis of osteoporosis. Lancet 1993 Mar 27; 341(8848): 797-801
   
   
2. Abelow BJ et al. Cross-cultural association between dietary animal protein and hip fracture: a hypothesis. Calcif Tissue Int. 1992 Jan; 50(1): 14-18
   
   
3. Messina M et al. The simple soyabean and your health. Avery Publishing Group, New York, 1994
   
   
4. Zhao et al. Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. Journal of Nutrition 2005; 135(10), 2379-2382
   
   
5. Setchell KD, Lydeking-Olsen E. Dietary phytoestrogens and their effect on bone: evidence from in vitro and in vivo, human observational, and dietary intervention studies. Am J Clin Nutr 2003;78:593S-609S.
   
   
6. Potter SM, Baum JA, Teng H, Stillman RJ, Shay NF, Erdman JrJW. Soy protein and isoflavones: their effects on blood lipids and bone density in postmenopausal women. Am J Clin Nutr 1998;68:1375S-1379S.
   
   
7. Alekel DL, Germain AS, Peterson CT, Hanson KB, Stewart JW, Toda T. Isoflavone-rich soy protein isolate attenuate bone loss in the lumbar spine of perimenopausal women. Am J Clin Nutr 2000;72:844-852.
   
   
8. Chen YM, Ho SC, Lam SS, Ho SS, Woo JL. Beneficial effect of soy isoflavones on bone mineral content was modified by years since menopause, body weight, and calcium intake: a double-blind, randomized, controlled trial. Menopause 2004;11:246-254.
   
   
9. Lydeking-Olsen E, Beck-Jensen JE, Setchell KD, Holm-Jensen T. Soymilk or progesterone for prevention of bone loss--a 2 year randomized, placebo-controlled trial. Eur J Nutr 2004;43:246-257.
   
   
10. Zhang X, Shu XO, Li H, Yang G, Li Q, Gao YT, Zheng W. Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women. Arch Intern Med 2005;165:1890-1895.
    
    
11. Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake inhibits bone resorption and stimulates bone formation in menopausal women: meta-analysis of randomized controlled trials. Eur J Clin Nutr 2008;62:155-61.
    
    
12. Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: Meta-analysis of randomized controlled trials. Clin Nutr 2008;27:57-64.